1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to systems and methods for chemical vapor deposition (CVD) and thermal processing, such as epitaxial deposition.
2. Background
A variety of semiconductor processes require uniform thermal processing at high temperatures. An example of such a process is called chemical vapor deposition (CVD) in which a layer of a material from the vapor phase is deposited onto a semiconductor substrate having been placed on a susceptor within a reactor. The susceptor is then heated either by induction or high intensity light radiation to high temperatures, typically between about 800 to 1250xc2x0 C. Gases are then passed through the reactor and the deposition process occurs by chemical reaction, within the gas phase, but closely adjacent to the surface of the substrate. The reaction results in the deposition of the desired product onto the substrate.
One form of this type of processing is called epitaxy, in which a single-crystal layer of a substance is deposited onto a substrate that is also single-crystal in nature. As an example, silicon epitaxy is one of the first steps performed in the fabrication of an integrated circuit device, and in this process a layer of doped single crystal silicon is deposited onto a silicon wafer in order to have a layer of known and closely regulated resistivity into which transistors and other devices may be formed. Epitaxy offers a convenient method for controlling the thickness, concentration, and profile of the doping layer.
An important parameter that must be controlled during an epitaxial deposition is the temperature uniformity of the substrate. Non-uniformities in temperature of the substrate can lead to a process of plastic deformation called slip, in which the crystal relieves built-up stresses by allowing portions of its structure to move relative to other regions. Slip occurs in a crystal over certain crystallographic planes and along certain crystallographic directions, causing one portion of the material to be displaced relative to another. A common cause of slip in a crystal is a temperature gradient during film growth, but it can also be the result of the manner in which the substrate is supported, the mechanism by which the substrate is heated, and the time-temperature profile of the epitaxial process. Slip-related defects are most often found at the edges of a substrate and appear as short lines.
Thermal gradients in a substrate may arise as a result of a non-uniform thermal environment within the CVD reactor. Because there are gases flowing within a CVD reactor, heat transfer mechanisms involve conduction and convection as well as radiation. However, radiative heat transfer may well be the most important with regard to temperature uniformity. A substrate adjacent to a heated susceptor within a cold walled reactor will see a variety of thermal gradients in both axial and radial directions. These thermal gradients have a large effect because radiative heat transfer between two objects is a function of the two temperatures, each temperature taken to the fourth power.
In many CVD and epitaxial deposition systems, high intensity lamps such as tungsten-halogen lamps are used to selectively heat a wafer within a cold wall furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. However, it is more difficult to control the temperature of the semiconductor substrate using only low thermal mass lamp heating. Some reactors use a large thermal mass silicon carbide coated graphite susceptor to maintain temperature uniformity of the substrate during processing. The substrate to be processed is placed either on or adjacent to the susceptor, and because of the susceptor""s high thermal conductivity, it can conduct heat laterally to maintain temperature uniformity and even out non-uniformities across the substrate. The susceptor is typically wider than the substrate which allows it to compensate for radiative heat loss at the edge of the substrate.
Alternatively, the susceptor may be heated by RF induction. This method takes advantage of the fact that an oscillating electric current passing through a conductor placed adjacent to the susceptor produces an oscillating magnetic field around the conductor, which in turn induces an oscillating current in the susceptor itself. Since the susceptor has an electrical resistance, the oscillating electrical current causes the susceptor to heat up. It should be noted that the current induced in the susceptor falls off non-linearly with distance from the conductor. The relationship is that the magnetic flux varies as the inverse square of the distance.
A typical configuration of the coil profile is shown in FIG. 1. The distance between any one particular coil segment and the susceptor may be adjusted with standoffs (not shown in FIG. 1). The coil in FIG. 1 is profiled to compensate for the radiative heat losses that occur at the edge of the susceptor when the susceptor is at processing temperature, and thus, coil segment 120 is closer to the susceptor than coil segment 122. It should be noted that the coil configuration shown in FIG. 1 is optimal when the reactor is at processing temperature, but may not be not optimal during transient periods when the reactor is being heated or cooled.
What is desired is an improved apparatus and method for CVD and/or epitaxial processing of a semiconductor substrate. Preferably, such a system and method would provide a uniform substrate processing temperature such that temperature gradients in the substrate, and the resulting problems with defects such as crystallographic slip, are reduced or eliminated.
Aspects of the present invention provide a CVD reactor for epitaxial processing, the reactor configured to reduce thermal gradients in the substrates onto which epitaxial layers are being deposited. Reducing thermal gradients in a wafer diminishes slip. One type of epitaxial reactor comprises an RF induction coil positioned adjacent to a silicon carbide coated graphite susceptor. An alternating current through the coil segments produces an oscillating magnetic field around each segment, which in turn induces a current in the susceptor. Electrical energy associated with the induced current is converted into thermal energy, thereby heating the susceptor. The coil is supported by a number of support studs, and the different segments of the coil may be set at different heights, thus varying the distance separating coil segments from the susceptor. Conventional methods of addressing temperature uniformity in a susceptor comprise adjusting the coil segments such that they are closer to the susceptor at the susceptor""s inner and outer edges than at the center in order to compensate for the greater amount of heat loss from the edges. Furthermore, the susceptor has to be rotated during heating and processing to minimize temperature gradients caused by the coil.
A problem with conventional methods of addressing temperature uniformity is that coil/susceptor separation profile is configured to provide optimum temperature uniformity in the susceptor at the processing temperature. This profile is not optimal for the transient portions of the process, for example, the heat ramp-up and cool down, where the closely spaced coil segments at the susceptor""s edges cause the edges to overheat during ramping. Since the coil/susceptor separation profile is not easily re-configured, especially during processing, apparatus and methods for improving temperature uniformity in the susceptor during the transient portions of the process are needed, assuming the coil/susceptor separation profile is fixed.
An aspect of the present invention provides a mechanism for raising and lowering the susceptor while it is rotating within the reactor. Because the magnetic fields fall off non-linearly with distance from each coil segment, raising the susceptor de-couples the coils at the edge (which are closer to the susceptor) to a greater degree than the coils at the center (which are further away from the susceptor). As a consequence, the over-heating of the susceptor edge that would have occurred during ramp-up may be mitigated. Without this z-motion of the susceptor, the edge of the susceptor may be heated to a temperature as much as 40xc2x0 C. higher than the center. In one embodiment of the present invention, an algorithm is used to determine the desired distance between the coil and the susceptor to maintain uniformity at different temperatures. The susceptor is moved closer to the coil with each increment in temperature during the transient ramp-up period according to the algorithm. In addition, the susceptor may be moved away from the coil during a transient cool-down period in some embodiments. An additional advantage provided by the z-motion of the susceptor is that the reactor is more convenient to service, since it is easier for maintenance personnel to remove the susceptor from the reactor by first raising it.
Another aspect of the present invention provides for insulator shields that may be placed on top of the susceptor to compensate for those regions of the susceptor not thermally insulated by substrates. Heat loss from the susceptor occurs predominantly by radiation, and is proportional to the difference between (Tsusceptor)4 and (Tenvironment)4. The substrates themselves tend to provide insulation at the susceptor pockets which support the substrates, and shielding the remaining regions improves the temperature uniformity of the susceptor.
Another aspect of the present invention provides for additional heat shielding in the vicinity of the susceptor edges, both around and underneath the edges. These shields may be referred to as the inner and outer susceptor edge radiation shields and the bottom inner and outer circumference radiation shields, respectively. Since these shields reduce heat loss from the edges, the coil segments heating the edges may be spaced further away from the susceptor than they would have been otherwise, thus providing a more uniform separation profile.
Another aspect of the present invention provides for a thicker susceptor than is used in conventional reactors. The thicker susceptor allows for temperature variations at the bottom of the susceptor, caused by discrete coil segments, to even out as the heat is conducted through the susceptor to the top surface on which the substrates are supported. Conventional susceptor thicknesses are in a range such as 0.5 to 0.9 inches, and embodiments of the present invention provide for a susceptor thickness in the range 0.5 to 1.5 inches. In one aspect of the present invention, the susceptor thickness was increased from 0.9 to 1.2 inches.